The present invention relates to a method of separating and recovering components of organic compounds in water in a pervaporization process with the aid of membranes.
Mixtures of this type comprise, for example, organic compounds that are completely or partially saturated with water and are only partially water soluble, and which in addition contain water phases that are loaded with organic compounds, including emulsions, that are only slightly soluble therein. Mixtures of this type are formed, for example, as bottom sludge in fuel depots, in oil and solvent traps of waste water conduits, during various processes in the chemical industry, as reaction products during synthesis, in the circulation fluid of closed circuit fluid vacuum pumps, during high pressure or ultrasonic cleaning, during the cleaning of stationary and moving tanks, and during salvaging of liquids from tankers or similar vehicles that have been damaged or have been involved in an accident on water or on land.
In general, the desire is to remove the organic chemical compounds from the mixture, whereby on the one hand water having a high degree of purity and on the other hand the organic compounds having a high degree of purity are to be separated, so that the water can be supplied as usable water having an unlimited use, and the recovered organic compounds can be supplied for reuse for technical or commercial purposes.
The organic chemicals that are to be treated pursuant to the present invention include those having a low water solubility, i.e. aliphatic and aromatic hydrocarbons such as hexane, toluene or mixtures of these groups such as fuels for internal combustion engines; solvents having a limited water solubility, such as higher alcohols, esters, ketones, to the extent that they have a limited water solubility; ether; and other basic materials of organic chemistry, as well as of polymer chemistry, such as styrene.
The treatment of such mixtures is generally effected by separating the mixtures into a water phase, which is organically saturated, and into the organic phase, which is water saturated, whereby the separation process is itself effected by a settling or centrifuging process Pursuant to the heretofore known state of the art, the further processing of these phases, which cannot be used in this condition, is effected in various ways. For example, the organic compounds can have water removed therefrom via distillation, extraction, sorption, or extractive-medium distillation, whereby generally further byproducts result that in turn must be removed or otherwise treated; this is a significant drawback. In situations where it is particularly difficult to recover components, such mixtures are therefore pyrolytically or via high temperature combustion in special furnaces, converted, i.e. reduced, into secondary products that are to be handled. The water phase is similarly purified at high costs in the same process in separate units, or the concentration of the dissolved organic compounds is reduced by dilution with other waste water and is discharged in small quantities into the waste water system or, in the case of tanker ships, is pumped out into the sea. Due to the high ecological incompatibility, this latter method of removal will in the future be precluded by regulations.
It is fundamentally also known to carry out separations of this type via a pervaporization process with the aid of membranes. For example, U.S. Pat. No. 4,405,409 discloses the separation of organic liquids with the aid of distillation followed by pervaporization with the aid of membranes. In this connection, water as distillation sump is given off and the organic compounds from which water has been removed are removed as a product of the membrane pervaporization stages.
With another known separation process, for example pursuant to Rautenbach and Janisch (Chem. Ing.-Technik 59 (1987) No. 3, pp. 187-196), a method is described where a mixture of materials is separated via reverse osmosis in combination with pervaporization. Due to the high osmotic pressures, however, here also limits are placed on the water purification that cannot be overcome, so that the remaining residual quantity of solvent in the water is greater than 2,000 ppm. A further drawback of this heretofore known method is the completely different technical manner of operation of the thus combined processes, because the reverse osmosis operates at lower temperatures and extremely high pressure, whereas the pervaporization is a thermal process with vacuum evaporation on the back side of the membranes.
By way of explanation, pervaporization is a treatment or separation process for liquid mixtures. The mixtures that are to be separated are conveyed onto membranes that are largely impermeable to liquids; however, vapors can pass through the membranes in a controlled manner. A partial pressure is generally applied to the back side of the membranes in order to immediately withdraw vapors that pass through the membranes and to extract them as liquids at a condenser. However, it is also possible to withdraw the arriving vapors with the aid of a rinsing gas and to condense them at a condenser. Since the membranes are generally made of plastic, and the throughput capacity can be increased by reducing the thickness of the membranes, these very thin flexible sheets must be expediently secured or installed in special devices (pervaporization apparatus).
As indicated above, up to now liquid measures were predominantly separated by distillation. Unfortunately, with a number of mixtures, such as water/ethanol, chloroform/hexane, ethanol/cyclohexane, butanol/heptane, water/isopropanol, water/tetrahydrofuran, water/dioxane, methanol/acetone, methanol/benzene, ethanol/ethylacetate, and methanol/methylacetate, a concentration is often obtained that vaporizes azeotropically, i.e. produces a mixture that cannot be further separated in this form. This situation cannot be improved by the addition of further materials, whereby the treatment steps must then be continued by extractive distillation, which, of course, is associated with a considerable expense.
Pervaporization offers an alternative approach. With such a process, one of the two components is preferably absorbed by a membrane. The membrane, which is continuously supplied on the feed side with the mixture, is continuously discharged on the back side via the application of a vacuum, with the absorbed or adsorbed liquid being removed from the membrane as vapor, thereby making space for more liquid that is to be absorbed. Thus, by selecting suitable membranes, even azeotropic mixtures can be split up into their components.
This is the basis of the present invention, an object of which is to provide a method of separating and recovering the components of mixtures comprising organic compounds and water and having gaps in the range of miscibility, whereby with the inventive method not only the water but also the organic substances can be provided with such a high degree of purity that the impurities that remain are in each case less than 10 ppm.